: Alzheimer's disease (AD) is a common dementia or loss of cognitive abilities, which is linked to the degeneration of brain tissue. This proposal is directly focussed on determining whether complement activation in AD brain triggers a pathway leading to neuropathology and dementia. Investigation of the interrelationships between biochemical reactions (C' activation) that occur and the cellular responses triggered as a result of those biochemical reactions could lead to identification of regulatory points that can subsequently be targeted for therapeutic intervention for Alzheimer disease patients. This proposal will test the hypothesis that the activation of the classical complement pathway, while resulting in lytic destruction of cells within the brain, also triggers a local inflammatory response which may contribute to the neurodegeneration and subsequent cognitive dysfunction in Alzheimer's disease. Abundant immunochemical evidence of the activation of complement in AD brain has been reported. To further assess the in vivo significance of these events, a variety of defined synthetic b-amyloid peptides, designed to represent the in vivo peptides will be assayed, via a sensitive C4 consumption assay, for their relative ability to activate complement. The results will be correlated with the ability of these peptides to form fibrils (electron microscopy) as well as variations in the secondary structure (circular dichroism) between these peptides. In addition, the effect of other proteins known to be present in the AD senile plaque (clusterin and a-antichymotrypsin ) on this activation will be assessed. In a second series of investigations, we will investigate the activation of microglia, astrocytes and neurons, before and after stimulation with the complement activation fragment C5a and/or proinflammatory cytokines. Generation of superoxide, triggering of phagocytosis, secretion of proteases, and production of cytokines and complement proteins will be determined. The ability of b-amyloid, or b-amyloid complexed with the complement initiation protein C1q to induce these inflammatory activities will also be tested, initially using in vitro primary human cell cultures when possible or primary rat cell cultures when necessary and/or appropriate. The results of these experiments will clarify the factors regulating, and the role of, complement-mediated inflammatory mechanisms in the development of neurodegeneration and cognitive dysfunction in AD. In addition, findings here should increase the understanding of other complement-mediated pathology in the CNS. Finally, the development of the in vitro model described here will allow testing of potential drugs that may modulate deleterious activities in an effort to slow or stop progression of this disease.